The new `enforce_canonical_emails` site setting ensures that emails in the
canonical form are unique.
This mean that if `s.a.m+1@gmail.com` is registered `sam@gmail.com` will
not be allowed.
The commit contains a blanket "tag strip" (stripping everything after +)
it also contains special handling of a "dot strip" for googlemail and gmail.
The setting only impacts new registrations after `enforce_canonical_emails`
The setting is default false so it will not impact any existing installs.
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